Polyester-polysulfide-polythiol curing agent and epoxy resins cured therefrom

ABSTRACT

A POLYESTER-POLYSULFIDE-POLYTHIOL CURING AGENT WHICH IS THE REACTION PRODUCT OF A POLYOL AND A MIXTURE OF A MERCAPTOALKANOIC ACID AND A THIODIALKANOIC ACID. THIS CURING AGENT IS EMPLOYED TO CURE EPOXY RESINS.

"United States Patent Olhce 3,803,089 Patented Apr. 9, 1974 US. Cl.260-47 EC 6 Claims ABSTRACT OF THE DISCLOSURE Apolyester-polysulfide-polythiol curing agent which is the reactionproduct of a polyol and a mixture of a mercaptoalkanoic acid and athiodialkanoic acid. This curing agent is employed to cure epoxy resins.

This is a continuation-in-part application of our copending applicationfor patent having Ser. No. 888,910, filed Dec. 29, 1969, now abandoned.

This invention relates to curing agents. This invention also relates tocured epoxy resins.

Curing agents such as pentaerythritol tetra(3-mercapopropionate) areknown in the art to cure epoxy resin. Not only are these curing agentsexpensive, but in addition they possess an offensive odor which ishighly undesirable.

It now has been found that the reaction product of a polyol and amixture of a mercaptoalkanoic acid and a thiodialkanoic acid can beemployed to cure epoxy resins. Not only is the reaction mixture lessodoriferous than the curing agents known in the prior art, but epoxyresins cured with the polyester-polysulfide-polythiol reaction productof this invention demonstrate an improvement in lap shear strength, asshown in the examples.

Accordingly, it is an object of this invention to provide apolyester-polysulfide-polythiol reaction product.

Another object of this invention is to provide an epoxy resin cured withthe polyester-polysulfide-polythiol reaction product.

Other objects, aspects and advantages of this invention will becomeapparent to one skilled in the art upon consideration of the followingdisclosure and appended claims.

The mixture of mercaptoalkanoic acid and thiodialkanoic acid employed inthis invention can have in the range of 5 to 95 weight percentmercaptoalkanoic acid, preferably in the range of 60 to 80 weightpercent mercaptoalkanoic acid. The remainder of the mixture is largelythiodialkanoic acid, but a minor amount of analogous dithiodialkanoicacid can also be present, that is, as much as one mole ofdithiodialkanoic acid can be present for each two moles ofthiodialkanoic acid.

The mixture of mercaptoalkanoic acid and thiodialkanoic acid can beprepared by any means known in the art. For example, the mixture can beprepared by mixing a mercaptoalkanoic acid with a thiodialkanoic acid.The preferred means of preparing these mixtures is by hydrolyzing amixture of mercaptonitriles and thiodinitriles which can be producedfrom alkene nitriles and hydrogen sulfide according to processes knownin the art, such as the process of U.S. Pats. 3,280,163 and 3,280,164.For example, acrylonitrile is reacted with hydrogen sulfide to form amixture comprising largely 3-mercaptopropionitrile andthiodipropionitrile that is subsequently hydrolyzed without thenecessity of separation to form a mixture comprising largely3-mercaptopropi0nic acid and thiodipropionic acid which can subsequentlybe employed in this invention.

The mercaptoalkanoic acids which can be employed in this invention arerepresented by the formula HS(CR CO OH wherein R is hydrogen or alkylhaving from 1 to 5 carbon atoms, the number of carbon atoms in all Rgroups per molecule is no more than 10, and n is an integer in the rangeof 1 to 5.

Specific examples of mercaptoalkanoic acids that can be employed in thisinvention are: S-mercaptopropionic acid, Z-mercatpoacetic acid,6-mercaptohexanoic acid, 2,2,3,3,4,4,5,5,6,6-decamethyl-6-mercaptohexanoic acid,4-mercaptoethyl-2-pentylnonanoic acid, 2-(2-methylpropyl)-S-mercaptopentanoic acid, S-mercapto-7-methyloctanoic acid, and the like,and mixtures thereof.

The thiodialkanoic acids which can be employed in this invention arerepresented by the formula wherein the number of carbon atoms in all Rgroups per molecule is no more than 20, preferably not more than 10, andR and n are as previously defined.

Specific examples of thiodialkanoic acids which can be employed in thisinvention are thiodipropionic acid, i.e.,

3- l-carboxyethylthio) propanoic acid,

2-(1-carboxyethylthio) propanoic acid, or

3-(2-carboxyethylthio)propanoic acid,

2- (carboxymethylthio ethanoic acid,

6-(S-carboxy-1,2,3,4-tetramethylhexylthio)-2,3,4,5-

tetramethylheptanoic acid,

6-(5-carboxy-l,1,2,2,3,3,4,4,S-nonamcthylhexylthio)-2,2,3,3,4,4,5,5,6-nonamethylheptanoic acid,

4- 3-carboxypentylthio -2-isopropylbutanoic acid,

6- (5-carboxy-3pentyldecylthio -2,4-dipentylhexa11oic acid,

6-(5-carboxydecylthio)undecanoic acid,

5- (4-carboxy'3-methy1pentylthio -2- (Z-methylpropyl pentanoic acid,

4-(4-carboxybutylthio)pentanoic acid, and the like,

and mixtures thereof.

The polyols employed according to the invention have on average morethan 2, and generally at least on the average 2.5, preferably at least3, hydroxy groups per molecule. The polyols such as at least one dioland another polyolof higher functionality can be admixed to form polyolshaving an average of more than 2 hydroxy groups per molecule. Theproducts of the invention can be prepared by contacting at least onepolyol, as defined herein, with a mixture of mercaptoalkanoic acid andthiodialkanoic acid under suitable reaction conditions.

The polyols which are employed in this invention are represented by theformula Y(OH) wherein Y can be a hydrocarbon moiety having in the rangeof 2-40, ordinarily 3-40, carbon atoms per moiety, x is an integer of atleast 2 and ordinarily in the range of 3-20, and the number of carbonatoms per molecule is equal to or greater than x.

Specific examples of polyols that can be employed in this invention arepentaerythritol,

glycerin,

2-butene-1,4-diol, 1,4-cyclohexanediol, pyrocatcchol,

3 ,4-dimethylpyrocatechol, l-phenyl-1,2,3-propanetriol,

1,5 -naphthalenedimethanol, 2,3-dimethyl-1,4-naphthalenediol1,4,5,8-naphthalenetetrol,

3-hydroxymethyl-2-pentene-1S-diol,

1,3-propanediol,

1,2-ethanediol,

1,2,3,4-butanetetrol,

mannitol,

2-ethylhexane-1,3-diol,

1,20-eicosanediol,

1,40-tetracontanediol,

l,3,4,7,9,11,13,l5,17,l9,22,24,2618,30,32,34,36,38,40-

tetracontaneicosol,

2,2-di (p-hydroxyphenyl propane,

low-molecular weight hydroxy terminated polybutadienes,

low-molecular weight hydroxy terminated polyisoprenes,

1,3,6-hexanetriol,

3 ,3-di(p-hydroxyphenyl pentane,

sorbitol, and the like,

and mixtures thereof.

The mixture of mercaptoalkanoic acid and thiodialkanoic acid is reactedwith at least one polyol to form the polyester-polysulfide-polythiolreaction product of this invention by an esterification reaction whichcan be effected in any suitable equipment at temperatures preferably inthe range of about 50250 C.

The reaction can be elfected in the presence or absence of diluentswhich are substantially inert to the reactants and products under thereaction conditions employed. These diluents can comprise as much as 95weight percent of the reaction mixture, if desired.

Acid esterification catalysts, such as paar-tolnenesulfonic acid,benzenesulfonic acid, sulfuric acid, and the like, can also be employed,if desired. In that event, the diluents employed should also be inert tothese acid catalysts. Examples of diluents that can be employed includetoluene, benzene, xylene, cyclohexane, and the like, and mixturesthereof.

The reaction pressure normally ranges from 0.5 to 10 atmospheres. It isnormally desirable to provide a means of removing water of reactionduring the course of the esterification. This function can beaccomplished by means known in the art such as continuously condensingthe vapors and refluxing the liquid phase and collecting water of thereaction in a water trap.

The esterification reaction should be effected to such an extent that atleast 80 percent of the hydroxy groups of the polyol are reacted withcarboxylic groups of the mercaptoalkanoic acid or thiodialkanoic acid toform ester groups. In general, the mole ratio of hydroxy groups of thepolyol to carboxylic groups of the acids ranges from 0.8:1 to 1.221.Nearly stoichiometric amounts are preferred to form the ester linkagesof the desired polyesterpolysulfide-polythiol reaction product. Ingeneral, an average of more than 2, ordinarily an average of at least2.5, and preferably at least 3 ester groups are formed for each moleculeof polyol.

The polyester-polysulfide-polythiol reaction product of this inventioncan be employed to cure epoxy resins in a variety of formulations whichare useful as coatings, adhesives, and the like. These curablecompositions are normally prepared by contacting any epoxy resin withthe polyester-polysulfide-polythiol reaction product of this inventionin the presence of a suitable accelerator. Such contacting can be doneby methods well known to the art for conventional curing of epoxyresins. For example, the formulation can be mixed in the presence ofsuitable diluents and the resulting mixtures can be sprayed on thesurface which is to be coated or applied to surfaces which are to beadhered. Accelerator can be added to accelerate the curing reaction, ifdesired. For example, the epoxy resin and a solvent, if desired, can besprayed from one orifice or from a multi-orifice spray gun while theother orifice can be employed to spray a mixture comprising curingagent, accelerators, solvents, diluents, and the like. Otherconventional means of applying curable epoxy resin coatings or adhesivescan also be employed within the scope of this invention. Preferably,sufiicient quantities of epoxy resin and curing agents are employed soas to provide about one thiol group per epoxide group. However, a rangeof about 0.8 to 1.2 thiol groups per epoxide group can be employed.Catalytic amounts of accelerator are preferred. For example, the amountof accelerator ranges from 0.001 to 5 weight percent of the totalcombined weight of the epoxy resin and the prepolymer being employed.

The epoxy resins which can be cured according to this invention includeany conventional epoxy resin. Normally, those epoxy resins having atleast two terminal 1,2-epoxy groups per molecule are preferred. Forexample, resins of epichlorohydrin and bisphenol A are preferred. Theseresins are commonly made commercially by reacting epichlorohydrin withbisphenol A in the presence of a basic catalyst such as sodiumhydroxide. The second group of epoxy resins which can be employed inthis invention are the epoxy novolac resins. These are basically phenolformaldehyde novolac resins whose phenolic hydroxide groups have beenconverted to glycidyl ethers. Commonly, the average number of epoxygroups per molecule of such compounds is greater than 2. Generally,these compounds have epoxy functionality in the range of 3 to 8 epoxygroups per molecule. A third type of epoxy resins are the epoxy esters.Particularly preferred is diglycidyl isophthalate which is derived fromisophthalyl chloride and glyidol.

Amine accelerators can also be employed, if desired. Generally, allamines or mixtures of amines known in the art to be useful asaccelerators for the curing of epoxy resins with polythiols can beemployed. Tertiary amines are preferred. However, any amine that is anin situ precursor to a tertiary amine can also be employed. Examplesinclude 2,4,6-tris(dimethylaminomethyl)phenol, diethylenetriamine,phenylenediamine, tri-n-hexylamine, benzyldimethylamine, morpholine,triethylenetetraamine, and the like.

Suitable diluents that can be employed in this invention include anymaterial that does not deleteriously affect the curing or otherproperties of the epoxy resin coatings or adhesives. Specific examplesare methyl ethyl ketone, diethyl ether, tetrahydropyran, acetone,benzene, toluene, xylene, hexane, dioxane, tetrahydrofuran, and thelike.

The uncured coatings or adhesive compositions of this invention, afterapplication, can be cured in any conventional manner. Curingtemperatures range from 0 C. to C. Normally, elevated temperatures arenot required to cure the epoxy resin compositions. Temperatures of 15-20C. are generally satisfactory.

Formulated compositions of this invention can be coated or adhered toplastics, metals, wood, ceramics, and the like. One preferredapplication is as a coating for polyolefins to prevent permeation byhydrocarbons. Gasoline tanks fabricated of polyolefins such as Marlex(trademark for polyolefin) and coated with apolyester-polysulfidepolythiol reaction product cured epoxy resin ofthis invention are impervious to permeation by hydrocarbons.

The advantages of this invention are further illustrated by thefollowing examples. The reactants and proportions and other specificconditions are presented as being typical and should not be construed tolimit the invention unduly.

EXAMPLE I A stirred reactor was charged with 424.0 g. of acrylonitrile,545.0 g. of hydrogen sulfide, 4 ml. of methylethylpyridine, and 8.0 g.of sulfur. The reaction mixture was agitated for five hours, with thereaction temperature being maintained between 2.5 and 65 C. Upondegassing, a product weighing 679 g. was recovered. A 100 g. fraction ofthe product ,was distilled at 0.5 mm. Hg to yield 71 g. ofmercaptopropionitrile product. This constituted a yield of 68.2 molepercent based on the acrylonitrile charged. The composition prepared wasdetermined to be comprised of 71 weight percent 3-mercaptopropionitrileand 27 weight percent of dicyanoethyl sulfide and disulfide (largelydicyanoethyl sulfide).

The 579 g. portion of the above product was hydrolyded to thecorresponding carboxylic acids as follows: 579 g. of the product, 11.6g. of p-toluenesulfonic acid, and 873 ml. of concentrated hydrochloricacid were charged to a reactor at 25 C. and then maintained at atemperature of 95 to 100 C. for 5 hours. A product was recovered whichcontained B-mercaptopropionic acid and mostly thiodipropionic acid in amole ratio of 81:19. The product, which weighed 1555 g., was found tocontain 32.2 weight percent of S-mercaptopropionic acid, 12.8 weightpercent of substantailly thiodipropionic acid, 23.9 weight percent ofammonium chloride, and 25.7 weight percent of water, hydrogen chloride,etc.

A 225 g. portion of the reaction product and 37.5 g. of pentaerythritolwere charged to a reactor and heated to 100 C. for 5 hours. During thistime 83 milliliters of water was removed. Toluene was added, andadditional water was removed. Chloroform was added and the mixture waswashed with a five percent aqueous solution of sodium bicarbonate withan added 25 g. of ammonium chloride to break the resulting emulsion. Theorganic layer was dried over magnesium sulfate and concentrated undervacuum to yield 99.0 g. of polyester-polysulfidepolythiol prepolymer.

A series of three more runs were made in a similar manner and acomposite was made of the polyesterpolysulfide-polythiol reactionproducts thus prepared. The composite had a molecular weight of 860, anacid number of 5.7, an equivalent weight of 202.5 based on the percentof SH groups present, and a SH group concentration of 15.8 percent.

This example demonstrates the preparation of thepolyester-polysulfide-polythiol reaction products of this invention.

EXAMPLE II A commercial epoxy resin, Epon 828 (a trademark for thereaction product of epichlorohydrin and bisphenol A) having a molecularweight of about 390 and an epoxide equivalent per 100 g. of 0.53 wasobtained from the Shell Chemical Company. A total of 5.70 g. of thisepoxy resin was blended with 6.06 g. of thepolyesterpolysulfide-polythiol reaction product of Example I and 0.125g. of an accelerator [2,4,6tris(dimethylaminomethyl) phenol] The aboveadhesive mixture was tested for adhesive lap shear strength inaccordance with ASTM test procedure D1002-53T. Briefly, the method canbe described thus: Two precleaned aluminum strips (1.000 in. x 4.000inch x 0.064 in.) are placed in a holder designed so that /z-inch ofeach strip overlaps the other. The space between the strips is filledwith the adhesive mixture to be tested (generally 5-10 mils thick) andcured (usually 24 hours at 25 C., and 4 hours at 150 C.). The forcenecessary to break the resulting bond in a tensile shear fashion is thendetermined using an Instron Model TI testing machine equipped with aspecial chamber for controlling the temperature of the bond. Tensileshear strengths are calculated in pounds per square inch (p.s.i.) ofbonded area.

The adhesive mixture was found to have an adhesive lap shear strength of3740 p.s.i. at 17 C. and 4183 p.s.i. at 25 C.

EXAMPLE IH A commercial pentaerythritol tetra-S-mercaptopropionateproduct was obtained from the Carlisle Chemical Company. A total of 13.2g. of this curing agent was blended with 19.0 g. of Epon 828 epoxy resinand a small amount of accelerator as in Example 11.

This adhesive mixture was tested as a control for lap shear strength inaccordance with ASTM 1002-53T, as was done in Example II.

This prior art adhesive was found to have a lap shear strength of 2877p.s.i. at -l7C. and 3507 p.s.i. at 25 C.

The control run of this example and the run of Example II demonstratethat epoxy resins cured with the polyester-polysulfide-polythiolreaction product of this invention have improved the lap shear strengthsat ordinary environmental temperatures.

Although this invention has been described in considerable detail, itmust be understood that such detail is for the purpose of illustrationonly and that many variations and modifications can be made by oneskilled in the art without departing from the scope and spirit thereof.

We claim:

1. A curable composition comprising:

(a) an epoxy resin having at least two terminal 1,2-

epoxy groups per molecule and (b) the polyester-polysulfide-polythiolreaction product (I) a polyol having on average more than two hydroxygroups per molecule and up to 20 hydroxy groups per molecule and up to40 carbon atoms per hydrocarbon moiety, and an acid mixture of (II) amercaptoalkanoic acid represented by the formula HS(CR COOH wherein R ishydrogen or an alkyl having from 1 to 5 carbon atoms, the number ofcarbon atoms in all R groups per molecule is no more than 10, and n isan integer in the range of l to 5, and

(III) a thiodialkanoic acid represented by the formula I-IOOC(CR--S(CR),,--COOH wherein the number of carbon atoms in all R groups permolecule is no more than 20, and R and n are as previously defined.

2. A composition according to claim 1 wherein said epoxy resin is thereaction product of epichlorohydrin and bisphenol A.

3. A composition according to claim 1 wherein the number of thiol groupsper epoxide group ranges from 0.8 to 1.2.

4. A composition according to claim 1 wherein the ratio of thiol groupsto epoxide groups is 1:1.

5. A composition according to claim 1 wherein said polyol ispentaerythritol, and said acid mixture is a mixture of3-mercaptopropionic and thiodipropionic acids.

6. A composition according to claim 1 wherein (a) is the reactionproduct of epichlorohydrin and bisphenol A and (b) is the reactionproduct of pentaerythritol and a mixture of B-mercaptopropionic andthiodipropionic acids, and further wherein the composition additionallycontains 2,4,6-tris(di'methylaminomethyl)phenol as an accelerator.

References Cited Chem. Abstract (C.A.), vol. 66, 1967, pp. 38652q-38653.

WILLIAM H. SHORT, Primary Examiner T. E. PERTILLA, Assistant ExaminerUS. Cl. X.R.

10647 R; ll7124 E, 132 BE, 138.8 E, 148; 260 2EC, 30.4 EP, 32.8 EP, 33.2HP, 33.6 EP, 59, 78.4 EP, 79 R, 410.5, 410.6, 478, 481 R

